1. Field of the Invention
The present invention relates generally to an improvement in the present inventor's method or technique, as claimed in the related art, of conditioning a gas sensor through the application of pulse discharges in order to condition mixed-potential gas sensors for detecting gases commonly found in combustion exhaust throughout a wider temperature range.
2. Description of the Related Art
The sensing element used in zirconia oxygen sensors is generally formed of an impermeable zirconia layer separating two porous electrodes, which are typically made of platinum metal. The electrodes are then used to measure the differential oxygen concentration between the measured gas on the outside of the sensor, and a reference gas, usually atmospheric, on the inside of the sensor. By measuring the voltage between two electrodes, the differential oxygen concentration can be calculated. Sensor design has a form of either thimble cell is shown according to the PRIOR ART in FIG. 1. Such a sensor 20 includes a zirconia substrate 21, a measuring electrode 22, a reference electrode 23, connecting leads 24, 25, and a pencil heater 28. The sensor design may further be in stacked planar layers as shown in FIG. 2 according to the PRIOR ART. Such a sensor 30 includes a porous protective layer 31, an external electrode 32, a sensor laminate 33, an internal electrode 34, reference air laminate 35, an insulation layer 36, a heater 37 and a heater laminate 38, with electrical current provided through connection contacts 39. In both cases sensor design include an internal heater to maintain sensor temperature in the desired temperature range.
An improved method for activating such sensors is described in the inventor's own related prior art reflected in U.S. Pat. No. 7,585,402 and recently awarded U.S. Pat. No 9,304,101. A typical schematic of the pulse discharge technique according to this PRIOR ART is shown in FIG. 3 and described below, which is a representation of the sensor conditioning in accordance with the previous invention. During the charging phases of the Pulse Discharge Technique or PDT (I and III) sensor was connected to the charging power source, during the discharging phases (II and IV) sensor was disconnected from the charging power source and kinetics of the sensor discharge was measured. FIG. 4 further shows applied voltage 41 and measured voltage 42 between the sensor electrodes in accordance with a prior art.
Sensor response to varying concentrations of the analyzed gas was measured by:                a. linearization of the sensor discharge data recorded during the pauses between charging pulses in my versus Log(t) coordinates; and        b. extrapolation of the linear regression line to the fixed pause time of 10 sec as shown in FIG. 5.This approach provided for accurate and low level detection of the NOx concentrations in the combustion exhaust. However, several limitations related to a limited NOx concentration measurement range and strong cross-interference from Oxygen concentration variations (for sensor operating temperature T>400° C.), were revealed, which are described based on the following examples.        